1. Field of the Invention
The present invention relates to a silica-filled, sulfur-vulcanizable rubber composition.
2. Prior Art
Most rubber compositions contain a reinforcing filler such as a carbon black or a combination of a carbon black with a light-colored or white filler such as a silica or a silicate. Rubber compositions comprising relatively large amounts of a silica or a silicate are well-known in the art and pneumatic tires with rubber treads made from such compositions are generally referred to as so-called xe2x80x9cgreen tires.xe2x80x9d These rubber compositions typically do not contain a carbon black, or only contain a small amount of a carbon black, typically 5 to 20 phr.
It is well-known in the art that the dispersion of silica in rubber, especially in green tire rubber compositions, presents a problemxe2x80x94due to poor interaction between the filler and the rubber and strong filler-filler interactionxe2x80x94and that mixing is difficult as well as power- and time-consuming. Poor dispersion of silica in rubber leads to deteriorated physical and dynamic properties of the vulcanizate, in particular to increased hysteresis and increased heat build-up. In order to improve the dispersion, it is conventional to use a silica coupling agent, such as bis-(3-triethoxysilylpropyl)tetrasulfide (Si-69), a silane coupling agent sold by Degussa. Silica and silica coupling agents are typically used in passenger car tire treads. The silica/silane coupling agent system improves the wear of the tire tread (i.e. improves abrasion resistance) and improves the dynamic properties of the rubber vulcanizate; in particular it reduces hysteresis, which can be translated into better rolling resistancexe2x80x94leading to fuel savingsxe2x80x94without a negative effect on wet grip.
However, when use is made of such known silica coupling agents, the dispersion still is not optimal under standard mixing conditions. Furthermore, as a result of using a silane coupling agent volatile alcohols are formed, which escape from the rubber composition and present an environmental problem. In addition, a reduction of the amount of silica coupling agents is desired, since their use in conventional amounts adds to the costs of the rubber vulcanizate.
The present invention provides a solution to the aforementioned problems.
In accordance with the primary embodiment of the present invention, there is provided a silica-filled, sulfur-vulcanizable rubber composition comprising an unsaturated rubber, 20 to 100 phr of a silica, 1.6 to 8 phr of a silica coupling agent, 0.05 to 5 phr of an antidegradant, 0.1 to 5 phr of a vulcanization accelerator, 0.1 to 10 phr of sulfur, and 0.5 to 5 phr of a quinone diimine, with the proviso that the antidegradant is not a quinone diimine.
Other embodiments of the invention encompass details about specific composition ingredients and additives, the use of the composition of the invention in rubber compositions and products obtained thereby, all of which are hereinafter disclosed in the following discussion of each of the facets of the invention.
In the context of the present invention, the term xe2x80x9csilicaxe2x80x9d comprises silicas and silicates.
In this application, the abbreviation xe2x80x9cphrxe2x80x9d means the number of parts by weight per 100 parts by weight of rubber. In the case of a rubber blend, it is based on 100 parts by weight of total rubber.
Quinone diimines are known compounds in the rubber industry. WO 99/20687 relates to the high-temperature mixing of elastomeric material in the presence of a quinone diimine antidegradant and carbon black, prior to vulcanization. It is described that by mixing the quinone diimine with the elastomer prior to vulcanization, enhanced handling and processing qualities are realized in the vulcanized rubber.
Up to now quinone diimines have only been reported for use as antidegradants. Their use in silica-filled rubbers, in particular green tire rubbers, has not been disclosed or suggested in the art of rubber technology.
Any quinone diimine may be used in the silica-filled, sulfur-vulcanizable rubber composition of the present invention. Quinone diimines are known compounds in the art of rubber technology. Preferably, a p-quinone diimine is used in accordance with the present invention.
Preferably, the quinone diimine to be used in accordance with the present invention is selected from the group consisting of N-isopropyl-Nxe2x80x2-phenyl-p-quinone diimine, N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-quinone diimine, N,Nxe2x80x2-bis-(1,4-dimethylpentyl)-p-quinone diimine, N,Nxe2x80x2-bis-(1-ethyl-3-methylpentyl)-p-quinone diimine, N,Nxe2x80x2-diphenyl-p-quinone diimine, N,Nxe2x80x2-ditolyl-p-quinone diimine, and N,Nxe2x80x2-di-xcex2-naphthyl-p-quinone diimine.
In the rubber composition of the present invention, preferably an amount of 0.5 to 3.5, more preferably 0.5 to 2, most preferably 0.5 to 1.5 phr of a quinone diimine is used.
The rubber that is used in accordance with the instant invention is an unsaturated rubber. Preferably, the rubber is selected from the group consisting of styrene-butadiene rubber (SBR), butadiene rubber (BR), natural rubber (NR), isoprene rubber (IR), and mixtures thereof, such as a blend of SBR and BR. In green tires, typically solution polymerization-derived SBR is used.
The silica reinforcing filler that is used in accordance with the present invention is well-known to the person skilled in the art. The reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 277-294. Preferably, in the composition of the present invention a high-surface area silica or silicate, or a mixture thereof, is used. The silica-filled, sulfur-vulcanizable rubber composition in accordance with the present invention preferably contains 40 to 100, more preferably 50 to 90, most preferably 60 to 90 phr of a silica.
Any silica coupling agent may be used in accordance with the present invention. Preferably, a silane coupling agent is used.
The silica coupling agent is used in a conventional amount, i.e. 1.6 to 8, preferably 1.6 to 6, more preferably 3.2 to 6, most preferably 3.2 to 5 phr. In general, high-surface area silicas and silicates require more silica coupling agent than low-surface area fillers. Typically, for a silica having a surface area of 180 m2/g BET, 8 wt % (i.e. 6.4 phr) of Si-69xe2x80x94based on the weight of the silicaxe2x80x94is used.
In the composition of the invention sulfur, a sulfur donor or a mixture thereof is employed. The amount of sulfur to be compounded with the rubber usually is 0.1 to 10, preferably 0.1 to 5, more preferably 0.5 to 3 phr. If a sulfur donor is used, the amount thereof should be calculated in terms of the amount of sulfur.
Typical examples of sulfur donors that can be used in accordance with the present invention include dithiodimorpholine, caprolactam disulfide, tetramethylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. The reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 231-233.
In the composition of the invention either a single vulcanization accelerator or a mixture of accelerators can be employed. For vulcanization accelerators that can be used in accordance with the present invention the reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
Typical vulcanization accelerators include thiazole- and benzothiazole-based accelerators, for example 2-mercaptobenzothiazole and bis(2-benzothiazolyl) disulfide, benzothiazole-2-sulfenamide-based accelerators, such as N-cyclohexyl-benzothiazole-2-sulfenamide, N-tert-butyl-benzothiazole-2-sulfenamide (TBBS), N,N-dicyclohexyl-benzothiazole-2-sulfenamide, and 2-(morpholinothio)benzothiazole, thiophosphoric acid derivatives, thiurams, dithiocarbamates, diphenylguanidine (DPG), diorthotolyl guanidine, dithiocarbamyl sulfenamides, xanthates, and mixtures of one or more of these accelerators. Preferably, the vulcanization accelerator comprises a benzothiazole-2-sulfenamide. A combination of a benzothiazole-2-sulfenamide and diphenylguanidine is particularly preferred.
In the composition of the present invention the vulcanization accelerator usually is employed in an amount of 0.1 to 5, preferably 0.3 to 3, most preferably 0.5 to 2.5 phr.
In the rubber composition in accordance with the present invention an antidegradant is included. The antidegradant is not a quinone diimine. Examples of suitable antidegradants can be found in W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989 pp. 268-277.
An amount of 0.05 to 5, preferably 0.5 to 5, more preferably 1 to 3, even more preferably 1 to 2 phr of an antidegradant is used in the composition of the invention.
Preferably, the antidegradant is a p-phenylenediamine. Preferably, the p-phenylenediamine is selected from the group consisting of N-isopropyl-Nxe2x80x2-phenyl-p-phenylenediamine, N-(1,3-dimethyl-butyl)-Nxe2x80x2-phenyl-p-phenylenediamine (6PPD), N,Nxe2x80x2-bis-(1,4-dimethyl-pentyl)-p-phenylene-diamine, N,Nxe2x80x2-bis-(1-ethyl-3-methylpentyl)-p-phenylenediamine, N,Nxe2x80x2-diphenyl-p-phenylenediamine, N, Nxe2x80x2-ditolyl-p-phenylenediamine, and N,Nxe2x80x2-di-xcex2-naphthyl-p-phenylenediamine.
Conventional rubber additives may also be included in the silica-filled, sulfur-vulcanizable rubber composition in accordance with the present invention. Examples include processing oils, such as aromatic oils, tackifiers, waxes, (phenolic) antioxidants, antiozonants, pigments, e.g. titanium dioxide, resins, plasticizers, factices, vulcanization activators, such as stearic acid and zinc oxide, and fillers such as carbon black. These conventional rubber additives may be added in amounts known to the person skilled in the art of rubber compounding. The reader is also referred to the examples that are described below. As mentioned above, carbon black may be included in the composition of the instant invention, typically in an amount of 5 to 20 phr.
Further, vulcanization inhibitors, i.e. scorch retarders, such as cyclohexyl-thiophthalimide, phthalic anhydride, pyromellitic anhydride, benzene hexacarboxylic trianhydride, 4-methylphthalic anhydride, trimellitic anhydride, 4-chlorophthalic anhydride, salicylic acid, benzoic acid, maleic anhydride, citraconic anhydride, itaconic anhydride, and N-nitrosodiphenyl-amine may be included in conventional, known amounts. For further details on these typical rubber additives and vulcanization inhibitors, see W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
Finally, in rubber compositions for specific applications it may also be desirable to include steel cord adhesion promoters such as cobalt salts and dithiosulfates in conventional, known quantities.
A preferred silica-filled, sulfur-vulcanizable rubber composition in accordance with the present invention comprises styrene-butadiene rubber (SBR), preferably solution SBR, 40 to 100 phr of a silica, 1.6 to 6 phr of a silica coupling agent, more preferably a silane coupling agent, 0.5 to 5 phr of an antidegradant, 0.3 to 3 phr of a vulcanization accelerator, more preferably comprising a benzothiazole-2-sulfenamide, most preferably a combination of a benzothiazole-2-sulfenamide and diphenylguanidine, 0.1 to 5 phr of sulfur and/or a sulfur donor, and 0.5 to 3.5 phr of a quinone diimine.
The silica-filled, sulfur-vulcanizable rubber composition of the present invention is mixed and vulcanized in a conventional way, i.e. mixing and vulcanization are carried out using means and equipment that are well-known to a person skilled in the art. Suitable mixing and vulcanization procedures are described in W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.
The vulcanization process typically is carried out at a temperature of 110-200, preferably 120-190, more preferably 140-180xc2x0 C., for a period of time of up to 12, preferably up to 6, more preferably up to 3 hours, most preferably up to 1 hour.
The present invention also pertains to articles of manufacture, such as pneumatic tires, e.g., for passenger cars and trucks, and industrial rubber goods, which comprise the rubber vulcanizate obtained by vulcanizing the rubber composition in accordance with the present invention in a conventional way.
The present invention is illustrated by the following Examples.